A known sealing system of this type as disclosed in French Pat. No. 1,600,889 in the name of Commissariat a l'Energie Atomique is capable of operating at high temperature, in particular for the hot extrusion of metal billets. This system consists in providing the ram at that extremity which penetrates into the chamber of the press with two members which interengage with a small clearance and are capable of relative motion along a common axis of application of the thrust load on said members. A seal is imprisoned between said members in the vicinity of the periphery of these latter. One of said members is provided with a projecting annular portion defining a flat bearing surface having a small cross-sectional area which is applied against the seal and extends at right angles to the axis. The other member is provided with a bearing surface for the same seal opposite to the annular portion of the first member and with an inclined face which is also applied against said seal and directed towards the exterior of the member. The force applied by the members to each other under the action of the ram in opposition to the reaction of the billet and of a liquid medium usually consisting of a lubricant which are present within the press chamber results in partial creep of the seal within the clearance formed between the two members and also within the annular space formed between the periphery of said members and the internal surface of the press chamber. This gives rise to a multiplied flattening effort in compression which is higher in value than the thrust or compressive force of the ram in the ratio of the area of the flat bearing surface of the first member to the area of application of the compressive force of the ram on said first member.
The resultant deformation of the seal achieves the necessary degree of pressure-tightness with the internal surface of the press chamber, said pressure-tightness being subsequently maintained continuously throughout the extrusion operation. In particular, since the compressive force on the ram increases rapidly at a uniform rate, flattening of the seal in compression first takes place in accordance with the above-mentioned process with partial filling of the clearance spaces up to the moment when said force attains a value at which the billet contained within the press chamber which is subjected to the hydrostatic pressure of the lubricant begins to pass through the die. From this moment, the deformation of the seal remains unchanged; it should be noted, however, that the particular configuration of the components of the device and especially the directional force exerted by these latter on the seal as a result of the inclined bearing surface of the second member which is directed outwards makes it possible to absorb the irregularities or defects which are present in the surface of the chamber during extrusion or any variations arising from the thrust of the ram. Since the resultant of the force applied to the seal is in fact of higher value than the force applied to each member, the variations in state of surface or in thrust result only in a deformation or complementary "consumption" of the seal which is sufficient to prevent interruption of pressure-tightness at all times.
Since the difference in time between the initial instant of application of pressure to the seal and commencement of extrusion of the billet is not negligible, the stage during which deformation of the seal takes place between the two imprisoning members is sufficiently long to result in appreciable consumption of the seal which accordingly undergoes creep or plastic flow within the clearance spaces provided for this latter, that is, from the time of inception of the cycle up to the instant of commencement of the extrusion operation. This deformation of the seal is of such magnitude in practice that creep taken place within the space formed between the members and the internal surface of the chamber, not only towards the rear of said members with respect to the direction of displacement of the ram but even in the opposite direction. The overall consumption of the seal is considerable, which practically prohibits its re-use after a first extrusion cycle and the seal therefore has to be changed for each new operation. In point of fact, if the force to be applied to the seal must be capable of causing its plastic deformation at the initial instant in order to ensure positioning between the two compression members both within the space between these latter and the internal surface of the press chamber, this is of course subsequently no longer essential except when taking into account accidental variations in the state of surface or in the thrust or compressive force exerted by the ram as mentioned in the foregoing.